One of the UK's most influential Islamic leaders, who has helped counter extremism in the country's mosques, is accused of advocating attacks on the Royal Navy if it tries to stop arms for Hamas being smuggled into Gaza.

Dr Daud Abdullah, deputy director-general of the Muslim Council of Britain, is facing calls for his resignation, after it emerged that he is one of 90 Muslim leaders from around the world who have signed a public declaration in support of Hamas and military action.

Abdullah, who led the MCB's boycott of Holocaust Memorial Day, was a member of the Mosques and Imams National Advisory Board, the body endorsed by the government that trains imams and was set up to curtail the activities of extremist clerics. In January, he briefed the home secretary, Jacqui Smith, and communities secretary Hazel Blears on the situation in Gaza and its likely impact on social cohesion in the UK.

There were calls last night for the government and the MCB to condemn Abdullah's actions. "The British government should stop funding organisations such as the MCB and supporting events such as Islam Expo, which hosts scholars from Saudi Arabia and Pakistan who hold extremist views," said Irfan Al Alawi, international director of the Centre for Islamic Pluralism.

"If the MCB is serious about tackling extremism, it should immediately expel extremists such as Daud Abdullah from its own ranks," said Ed Husain, co-director of the Quilliam Foundation, a counter-extremism thinktank. "The man is a fanatic."

Abdullah's name appears as a signatory to a declaration in Istanbul last month that describes Israel's recent military campaign as "the manifest victory which Allah has granted us in the land of Gaza". It opposes the "so-called Arab peace initiative" and the Palestinian Authority and issues a series of obligations to the "Islamic Nation", calling on it to "carry on with the jihad and resistance against the occupier until the liberation of all Palestine".

Obligation six declares that Muslims must seek to open the crossings in Palestine so that "money, clothing, food, medicine, weapons and other essentials" can enter Gaza and Palestinians "are able to live and perform the jihad in the way of Allah Almighty".

It warns: "The closure of the crossings, or the prevention of the entry of weapons through them, should be regarded as high treason in the Islamic Nation, and clear support for the Zionist enemy."

The most contentious obligation instructs Muslims to attack foreign navies. In January, Gordon Brown offered Royal Navy resources to help monitor events in Gaza and to stop weapons being smuggled into the territory.

But, according to the Istanbul declaration, there is an obligation for "the Islamic Nation to regard the sending of foreign warships into Muslim waters, claiming to control the borders and prevent the smuggling of arms to Gaza, as a declaration of war, a new occupation, sinful aggression, and a clear violation of the sovereignty of the Nation". It continues: "This must be rejected and fought by all means and ways."

Husain accused Abdullah of "betraying the Palestinian people". He added: "As well as potentially endorsing terrorism against British troops, Abdullah shows total disregard for human life."

A spokesman for the MCB, which says it speaks for about 400 mosques and Muslim organisations, declined to comment. Abdullah did not respond to repeated requests for an interview.

Two other prominent British Muslims ' names also appear as signatoriesto the declaration: Mohammed Sawalha, an organiser of Islam Expo, the huge annual gathering of Muslims in east London, added his name to the list; and Sheikh Rashid al-Ghannoushi of the Tunisian an-Nahdhah party, who resides in the UK, also signed.

A spokeswoman for the Department of Communities and Local Government said: "We are aware of the conference held in Istanbul last month and are very concerned that the statement from the event calls for direct support for acts of violence in the Middle East and beyond. We are also aware that a senior member of the MCB may have been a signatory to this statement. If it is proven that the individual concerned had been a signatory, we would expect [the MCB] to ask him to resign and to confirm its opposition to acts of violent extremism."